Thermostat apparatus



Dec. 18, 1956 P. R. LEE 2,774,846

THERMOSTAT APPARATUS Filed April 12, 1954 BRAZED 'BRAZED BRAZED 5 5 24- Bmzao 64 INVE NTOR PAUL R.LE

PEG- 4 5"" TTORNEY United States Patent THERMOSTAT APPARATUS Paul R. Lee, Mansfield, Ohio, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., 2: corporation of Pennsylvania Application April 12, 1954, Serial No. 422,427

4 Claims. (Cl. 200-438) This invention relates to a stacked unitary thermostat assembly and particularly to the manner of securing a bimetal switch actuating arm therein.

This type of switch has a plurality of resilient bimetal and contact supporting arms which are held in relatively fixed relationship by clamping a portion of each arm, together with suitable insulators and spacers, to form a plurality of layers in a single stack.

The stack is usually assembled by passing a support, such as a post or tube, through openings in the several switch components and clamping them together by riveting the ends of the support or by otherwise exerting a force in compression on the stack parallel to the axis of the support.

The insulators may be annular mica or ceramic members. The clamping forces which hold the stack in a fixed relationship are of such magnitude that most of the ceramic insulators are fractured during assembly. Sometimes broken pieces of the ceramic members fall out of the stack, leaving a void therein. The mica members may be of non-uniform thickness and slightly deformable under the large clamping forces.

As a result of the non-uniform application of forces through the deformed or broken insulating members in the stack, the bimetal is not always uniformly held therein. In one switch it may be firmly gripped only very near the support, whereas in another switch it may be firmly gripped near the outer edge of the insulating members in the stack. This non-uniform clamping of the bimetals changes their efiective length depending upon the bimetal area that is rigidly clamped.

Inasmuch as the deflection of a bimetal strip, for a given temperature change, varies as the square of the length, it is important in switch production to be sure that the effective length of a bimetal strip is uniform from one switch to another. The eflective length of the bimetal strip as referred to herein is the length of that portion of the bimetal strip which may deflect due to temperature changes of the bimetal. The switch characteristics vary considerably if even a small area of the bimetal, that is intended to be held rigidly within the stack, is permitted to deflect in response to temperature changes.

Thermostats of the stack type may be used to provide control at a single temperature, in which case the thermostat adjusting or calibrating means is fixed in a position determined during calibration. In this type of thermostat, a change in the effective length of the bimetal strip after calibration would change the temperature setting of the thermostat and require that the thermostat be recalibrated.

In thermostats of the type that may be adjusted during use to operate at various temperatures within a predetermined range, the temperature differential for a predetermined movement of the adjusting means is dependent upon the efiective length of the bimetal strip. A change in this effective length not only shifts the limits of the temperature range of adjustment but it also changes the Patented Dec. 18, 1956 temperature dilferential for a given movement of the adjusting means within this range. If this type of thermostat were used with an operating knob having indicia corresponding to the temperature calibration of the thermostat, the indicia could not accurately indicate temperature over the entire range of adjustment, even after calibration, unless the effective length of the bimetal were maintained constant or unless the indicia were changed slightly on each thermostat to correct for the changed temperature settings due to changes in elfective length of the bimetal from one thermostat to another.

Non-uniformity in the eifective length of bimetals in stack type thermostats may result during production, in which case the problem may be partially overcome during the calibration of the thermostat. However, once the thermostat is calibrated and installed, for instance in an electric appliance, any change thereafter in the effective length will prevent the thermostat from operating in the desired manner according to the temperature settings indicated by a control knob. This problem has been overcome in the present invention by rendering the portion of the bimetal which is held within the stack nondefiectible in response to temperature changes.

Accordingly, it is an object of this invention to provide a stacked type unitary thermostatic switch in which the effective length of a creep-acting bimetallic strip is fixed.

A further object of this invention is to provide a method of assembling unitary thermostatic switches in which the creep-acting bimetallic contact actuating strips have predetermined and uniform effective lengths.

In accordance with this invention, the bimetal strip in a stack type thermostat is brazed to a metal member in the stack having sufficient stiffness to render the brazed portion of the bimetal non-deflectible in response to temperature changes within the normal operating range of the thermostat, regardless of the non-uniformity in the application of clamping forces on the bimetal strip.

These and other objects are efiected by my invention as will be apparent from the following description taken in connection with the accompanying drawings, forming a part of this application, in which:

Fig. l is a side elevation of a first embodiment of this invention, showing a unitary thermostatic switch partially in section;

Fig. 2 is a perspective view of a sub-assembly of the embodiment shown in Fig. 1;

. Fig. 3 is a side elevation of a second embodimentof this invention, shown partly in section;

Fig. 4 is a side elevation of a sub-assembly of the first embodiment indicating the effective length of a bimetal switch actuating member; and

Fig. 5 is a sub-assembly of a switch made in accordance with the prior art and indicating the efiective length of a bimetal switch actuating member.

Referring now to Fig. 1, there are shown contact supporting resilient arms 10 and 12 and a bimetallic contact actuating arm 14 secured in stacked relationship in a unitary thermostatic switch. Relatively movable contacts 16 and 18 are attached near the free ends of contact arms 10 and 12 respectively. A ceramic insulating button 20 is secured to the free end of the bimetallic arm 14 and engages the outer end of the contact arm 10 to move it upward to separate contacts. 16 and 18 when the bimetal 14 deflects in response to an increase in its temperature to a predetermined value.

A tubular member or post 22 supports the contact arms 10, 12 and 14 in the thermostat stack and extends through openings in the fixed ends of these arms. The lower-end of the tubular member is provided with an en circling plate or flange portion 24 having a flat upper surface. The fixed end of the bimetal arm 14 lies-in engagement with the fiat upper surface of the flange 24. The under or high expansion side of the bimetal 14 is brazed to this flat surface over the entire opposed area of these two parts. This brazed area extends completely around the member 22. The flange 24 is of sufiicient stiffness so that the area of the bimetal which is brazed thereto will be incapable of deflecting in response to temperature changes within the normal operating range of the thermostat.

The metal flange 24 and the brazed area of the bimetal 14 should extend in the direction of the free or deflectible contact actuating end of the bimetal at least to the edge, nearest this free end, of that bimetal area which may be subjected to variable clamping forces in the switch stack. Thus, the brazed area should extend at least to the extreme right edge of a ceramic spacer 36 that bears on the upper surface of the bimetal in the illustrated embodiments of this invention.

By brazing the bimetal to the flange 24 better heat transfer is secured between these two parts, so that when the supporting member 22 is held in intimate contact with a supporting base, the temperature of which is being sensed, the bimetal will more closely follow this temperature.

The fixed ends of the contact arms and 12, together with their respective electrically conducting terminal members 26 and 28, are arranged on the supporting member 22 above the low expansion side of the bimetal 14.

Suitable annular insulating members, such as the mica disc 34 and the ceramic spacers 36, are provided in the stack assembly to insulate the contact arms 10 and 12 from each other and from the supporting member 22. In the end of the stack opposite from the bimetal arm, there is supported a plate which in turn supports a thermostat adjusting screw 32.

The upper end of the tubular supporting member 22 is deformed outwardly during assembly to form a flange 38 which engages the upper surface of the plate 30. After the forming of this flange 38, the stacked switch components are held in compression between this flange and the flange 24 to secure them in a fixed relationship.

The position of the adjusting screw 32 determines the temperature of the thermostat at which the contacts 16 and 18 will be separated. An insulating pin 40 is secured in the lower end of the adjusting screw 32. This pin passes through an opening in the contract arm 10 and bears against the upper surface of the contact arm 12. The latter is biased upwardly so that it will always remain in engagement with the pin 40 as the adjusting screw is rotated. The contact arm 10 is biased downwardly so that contacts 16 and 18 remain in engagement at all times except when separated by the upward deflection of the bimetal 14. The downward spring force of the contact arm 10, however, is not sufficient to move the contact arm 12 out of engagement with the pin 40.

The range of temperature adjustment in the thermostat shown in Fig. l is determined by the position of the lug 41 which is fixed to the adjusting screw. This lug engages a stop member 42 integral with the plate 30 to limit the rotation of the adjusting screw.

The thermostat assembly described above may be attached to any supporting base 44, for example a soleplate of an electric iron, by merely passing a screw through the tubular supporting member 22 to hold the flange 24 intimately in contact with the base 44. This mounting not only provides a firm support but also good heat transfer between the flange 24 and the base 44.

The embodiment of this invention shown in Fig. 2 is substantially the same as that shown in Fig. 1 with the exception that the bimetal is brazed to a flat plate which is not integrally formed with the tubular support ing member 52. The plate 50 may be an annular metal washer having. a flat upper surface to which the bimetal is brazed and this plate may be secured at any desired point in a stacked thermostat assembly. As shown in this embodiment, the various stacked switch components are held in compression in a fixed relationship by the flanges 54 and 56 on the ends of the tubular supporting member 52.

Stacked type thermostats are assembled with clamping forces that are so large that many of the ceramic insulators therein are broken; in fact, according to one scheme heretofore disclosed, all are deliberately broken. If the bimetal is clamped contiguous to a ceramic insulator, the fracturing of this insulator is very likely to change the effective bimetal length. If metal washers or other metal reinforcements are provided on both sides of the bimetal, the problems arising from changes in the effective bimetal length are reduced. However, the deformation or fracturing of the insulating members which occurs during manufacture, or after the thermostat is calibrated, often results in a concentration of forces on these washers causing non-uniform clamping of the bimetal members and non-uniformity in the effective lengths of the bimetal strips.

Fig. 5 is an exaggerated view of a bimetal clamped between a metallic washer 60 and the flange 64 on a stack supporting member 62. If, for example, the clamping forces are concentrated near the center of the washer 60, as shown in this figure, the effective length L of the bimetal member may be substantially increased. However, in practicing this invention, the effective length of the bimetal will remain constant as shown at L in Fig. 4. Furthermore, washers, such as the washer 60 shown in Fig. 5, which were previously found desirable, may be dispensed with when the bimetal is brazed to the flange 24.

The temperature range of adjustment established during manufacture by the location of the lug 41 on the adjusting screw will not be altered when using this invention. This eliminates expenses which occur in replacing or recalibrating thermostats if the effective length of the bimetal is changed during the stack-up riveting operation of the flanges 33 and 56, when not only the pressure of ceramic washer 36 against bimetal 14 varies, but also the location of the pressure varies. This pres sure variable may be due to the extremely variable fractures of the ceramic insulating washers plus slight variations in the relative fits of parts in the stack-up. In addition, the forces exerted by screw 46, when the thermostat is installed, will impose another added variable.

The term brazed as used throughout the specification is to be distinguished from welding. In the brazing operation carried out in accordance with this invention, neither the surface of the bimetal nor the surface of the flange is fused during the bonding process. By brazing the bimetal to the flange a firm bond between these members may be secured without the possibility of the bimetal being detrimentally effected as might be the case if this bond were efliected by welding. In welding, the surface of the bimetal would be fused to bond it to the flange and in so doing the areas of the bimetal near the edge of the flange may be so affected as to change the operating characteristics of the bimetal or weaken the latter so as to shorten its life.

While the invention has been shown in several forms, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit thereof.

What is claimed is:

1. In a unitary thermostatic switch of the type in which contact supporting arms and a bimetal contact actuating strip together with a plurality of insulating members are clamped in a stacked relationship and in which the clamped portion of said bimetal may be subjected to variable clamping forces in the stack, the improvement comprising a flat metal plate secured contiguous the clamped portion of said bimetal strip in said stack, said bimetal strip and said plate being brazed together over their entire contiguous surfaces to limit the contact actuating deflection of said bimetal in response to temperature changes to that bimetal portion which extends beyond said plate, the deflective contact actuating portion of said bimetal lying outside the clamped bimetal portion.

2. A unitary thermostatic switch comprising a plurality of contact supporting and contact actuating arms, each of said arms having a supporting portion, said arms being positioned with said supporting portions clamped to form a plurality of layers in a stacked relationship, insulating members in said stacked relationship for electrically insulating at least one of said arms, one of said arms comprising a bimetal strip having an area of its supporting portion that may be subjected to variable clamping forces in the stack, and a metal member in said stack having a fiat surface brazed to the supporting portion of said bimetal arm, said metal member having suflicient stiffness to prevent the brazed portion of said bimetal from deflecting in response to temperature changes within the normal operating range of said thermostatic switch, the brazed portion of the bimetal strip extending at least to the edge, of said clamped area, nearest the deflective contact actuating end of said strip.

3. A unitary thermostatic switch comprising a plurality of resilient contact supporting and contact actuating arms, each of said arms having a supporting portion forming a layer in a stack of switch components, means for insulating at least one of said arms in said stack, said insulating means including at least one ceramic insulating member, one of said arms comprising a flat bimetal strip, a flat metal member contiguous the sup- 35 porting portion of said bimetal strip, said member and the supporting portion of said bimetal strip being brazed together, clamping means for holding said arms fixed in said stack, said clamping means exerting forces in compression on said ceramic insulating member and the supporting portions of each of said arms, said bimetal strip having an area of its supporting portion that may be subjected to variable clamping forces in said stack, the brazed portion of said bimetal strip extending at least to the edge of said area nearest the deflective con tact actuating portion of the strip.

4. A unitary thermostatic switch comprising a plurality of contact supporting and contact actuating arms, each of said arms having an opening in one end thereof, said arms being positioned in a stacked relationship with said openings in alignment, annular insulating members in said stacked relationship, one of said arms comprising a bimetal strip, a metal member having a fiat surface adjacent said one end of said bimetal arm, said metal member and a portion of said bimetal arm adjacent the opening therein being brazed together, and a post extending through said openings and through said insulating members to rigidly clamp said arms, said insulating members and said metal member in said stacked relationship, the brazed portion of said bimetal arm and said member extending at least to the edge, nearest the deflectible contact actuating end of the bimetal, of that area of said bimetal that may be subjected to variable clamping forces in said stack, said bimetal strip being stiffened over the brazed portion so that its efiective deflective length is constant regardless of changes in the clamping forces in said stack during or subsequent to manufacture of said switch.

References Cited in the file of this patent UNITED STATES PATENTS 

